Capillary gas chromatography (GC) is a widely used method for separation and identification of analytes, or derivatives thereof, that are stable in the gas phase. Samples are typically introduced to the instrument as liquids, by syringe injection through a rubber septum, into a glass-lined chamber within a heater block that is fixed to a wax-lined capillary column. The liquid sample vaporizes in the inlet liner, mixes with the carrier gas and all, or a portion, of the gaseous sample is swept onto the capillary column. Within the column different compounds dissolve in the thin, liquid, waxy stationary phase on the capillary wall to differing degrees and their progression through the column proceeds a different rates as a result. The outlet end of the capillary column is attached to a detector, e.g. a flame photometric detector.
A principle concern to the efficiency of GC is rapid and uniform sample vaporization and introduction to the column. A typical, split flow and direct injection sample vaporization chamber is shown in FIG. 1, where a heated block 1 houses an injection port liner 3. Injection port liners are most typically made of borosilicate glass for ease of heat-forming the conical restriction 7 (also known as a radial taper) within which a capillary separation column end 5 finds purchase at a taper diameter that approximately matches the capillary outer diameter. The liner 3 is also equipped with a vent 17 (e.g., a hole through the glass wall) for splitting off some of the vaporized sample flow.
Carrier gas is introduced through an inlet port 19 and flows as the dashed arrows indicate: into the open end 23 of the liner 3, about the needle 11 of the sample introduction syringe (not shown), with some flow continuing distally along the liner bore and into the open end 7 of the column 5 within the tapered restriction of liner bore while flow also splits, flowing through the vent 17, about the outer diameter of the liner 3 and out the split-flow exhaust port 21. The capillary column is traditionally held in its physical location by a Swagelok connector in the GC oven wall, represented by the graphite ferrule 15, and the liner is typically centered and sealed within the heater block 1 by a Viton O-ring 13 and graphite gasket 2.
The heated block 1 is typically heated to approximately 200° C. to 300° C. prior to sample introduction until the glass liner 3 is equilibrated with the block temperature. The sample is introduced by plunging the syringe needle 11, through a rubber septum 9 and depressing the syringe plunger (not shown), ejecting sample from the beveled needle tip 25. Ideally, the sample vaporizes immediately, filling the open area of the liner 3 with a uniform and representative solution of sample in carrier gas that is swept onto the column 5 opening 7 within the liner 3 conical taper. The amount of sample that is introduced to the column may be varied by controlling the carrier gas split flow via an adjustable flow restrictor in the exhaust port 21 or by other means.
Sample interactions with the inlet liner or sleeve surfaces are problematic but are considered unavoidable. The sample may degrade by interaction with the borosilicate glass or with constituents carried on the liner. Notably, liners are not replaced or cleaned after each use: as this would be prohibitively expensive in terms of liner costs and instrument downtime. Compounds from previous injections, reversibly absorbed to the inlet liner, can release and result in spurious peaks or baseline drift. Compounds that irreversibly absorb may become active or reactive sites for interactions with subsequently studied compounds or may degrade, producing spurious peaks.
Accordingly, borosilicate liners are almost universally coated to mask the intrinsic surface activity and reactivity. Common “deactivation” methods include reacting the exposed (surface) silanol with organosilane reagents (e.g., bis(trimethylsilyl)amine). Other treatments using gaseous silane and derivatives thereof have also proven effective but deactivation coatings are temporary and simply mask the underlying reactivity.
Additional adverse activity is often knowingly introduced by liner manufacturers in the form of markings on the liners, usually as enamel glazes that contain transition metals and other active and reactive functionalities. The markings are claimed to be necessary for “identifying and tracking liners” or “for proper installation orientation” or “positioning packing materials.” While these markings are on the outer surface of the liner, the added activity and reactivity still interfere by way of diffusion of degradation products or reversibly absorbed compounds into the sample stream over time, particularly in split flow injection.
A less problematic method of marking liners has been glass etching or ‘frosting’, either by chemical or physical means. Etched surfaces are high in surface area (increasing total activity) and may be saturated with silanol groups, absorbed etching process contaminants, etc. and etching provides only low resolution such that the markings are typically large. Other purposed schemes for marking liners have also been proposed, e.g. U.S. Pat. No. 8,366,814 (Jones, et al.), proposes indicating compounds for visual determination of the liner temperature (for safety in hot swapping) or prior wear or abuse (potentially degraded deactivation due to exposure to excess temperature, oxygen or moisture, for example, and U.S. Pat. No. 8,999,044 (Rohland, et al.) proposes using color coding via use of colored glasses in liner construction.